This invention, referred to herein as chrominance smoothing, relates to a method for reducing the entropy in a luminance-chrominance-chrominance (L,C,C) color-space representation of a digital color image, with a resulting (after reduction) minimal degradation occurring in visually perceived image quality. Practice of the invention which, as will be explained, can be implemented in a number of different specific ways, takes special advantage of the human visual system's relatively low sensitivity to chrominance differentiation between adjacent image pixels at and near the extremes (the black or white regions in an image) of luminance values. A preferred manner of implementing and practicing the invention focuses attention on the low-luminance extreme—namely, the (“black” and near-black) regions in an image.
As is well known in the relevant art, manipulation and handling of color digital imagery to achieve a practical balance between retained, high image quality and desired, reduced data-handling file size, bit rate and bandwidth, is a heavily attended area of technical attention and development. For example (just one among many possible examples), color imagery, both still and motion, is typically acquired in colorimetric R,G,B (red, green and blue) color space in which the color of a pixel is described by values assigned to these three (R, G, B), components. Image-data management in the form of image compression often, with regard to such an acquired category of color imagery, begins with a color-space conversion to create an appropriate luminance-chrominance-chrominance (L,C,C) color-space (channel) representation, such as Y,u,v Y,Cb,Cr, or L*, a*, b*—a representation in which the low human visual sensitivity to high-frequency chrominance information is exploited. This conversion, which provides an opportunity for chrominance sub-sampling, thus promotes the opportunity for significant reduction in data-handling factors, such as bit rate and bandwidth. Such a conversion is accomplished fundamentally by reducing the resolution of the chrominance images—u and v in Y,u,v, Cb and Cr in Y,Cb,Cr, and a* and b* in L*, a*, b*.
The present invention springs from this kind of L,C,C conversion and data-handling improvement, and further significantly advances this area of improvement by recognizing, and taking advantage of, the fact, mentioned briefly earlier, that the human visual system's sensitivity to chrominance differentiation at and near the extremes of luminance is quite low. The basic approach proposed by the methodology of the invention is to capitalize on this diminished sensitivity by smoothing, or evenizing, and greatly minimizing chrominance pixel-edge differentiation, thus to create an appreciable reduction in chrominance-channel entropy, and to improve compression of the chrominance channels. Thus, the method proposed by the invention uses the fact that, at the extreme luminance values (black or white regions) of an image, the chrominance information is less significant to perception. As will be seen, this method may be used in conjunction with chrominance sub-sampling to enable further image compression.
While a number of specific and useful entropy-reduction approaches, well within the scope of this invention, can be employed to implement the practice of the invention, only several of these are specifically illustrated and/or described herein to demonstrate the special utility of this invention. Further, while practice of the invention can produce significant chrominance entropy reduction at or near either one of the two luminance extremes, I have found that a preferred approach involves applying the invention at or near a luminosity region associated with the low (black-end) luminance extreme. Regions of an image containing pixels with low luminance values are typically text regions, often black text on a colored or textured background. As such, both the luminance and chrominance channels of the related image contain edges which may be “smoothed”, and it is such smoothing which this invention addresses.
These and other objects and advantages which are attained by the present invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.